This invention is a method for increasing the efficiency of xylene isomerization in a process for the catalytic conversion of ethylbenzene coupled with xylene isomerization. More specifically, the invention increases the efficiency of xylene isomerization by using a two stage isomerization process.
Xylenes and ethylbenzene are C.sub.8 benzene homologues having the molecular formula C.sub.8 H.sub.10. The three xylene isomers are ortho-xylene, meta-xylene and para-xylene, which differ in the positions of two methyl groups on the benzene ring. The term mixed xylenes describes a mixture of ethylbenzene and the three xylene isomers. Mixed xylenes are largely derived from petroleum.
Para- and ortho-xylenes are valuable chemical intermediates. In the petrochemical complex, they are produced in large part by recovery of these compounds from both the crude distillation and the C.sub.8 heart cut of the reformer. Recovery is accomplished via several selective separation processes, such as C.sub.8 heart-cut distillation to yield high-purity ortho-xylene "ortho-splitter"), and selective crystallization or sorption processes. The remainder from these separation technologies involves mostly meta- and ortho-xylenes, ethylbenzene, and benzene, with relatively low concentrations of other aromatics.
Several technologies exist for upgrading the chemical value of this recycle stream. For example, there are commercial processes for converting this stream into one containing an equilibrium mixture of xylenes--that is, roughly 50 weight percent (wt %) meta-xylene, and 25 wt % each of para- and ortho-xylenes--via isomerization. These processes also reduce the ethylbenzene concentration in this recycle loop through mechanisms such as cracking, or hydrocracking (hydrogenation of the ethylene thus formed from ethylbenzene cracking by means of a functional metal).
Although these processes have been practiced commercially for many years, great improvements could be realized if it were possible to significantly reduce the capital costs of the isomerization complex. The commercial processes that are presently being used require large capacity processing vessels to produce a relatively small amount of product. Therefore, it is desirable to find a method that increases the amount of product that can be produced by existing equipment and also allow new plants using smaller capacity equipment to produce the equivalent amount of product.
The various xylenes may be derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C.sub.8 aromatics in such mixtures and their properties are shown below:
______________________________________ Freezing Boiling Density Equilibrium* Point (.degree. C.) Point (.degree. C.) (Kg/m.sup.3) Proportion (wt %) ______________________________________ Ethylbenzene -95.0 136.2 869.9 8.5 Para-xylene 13.2 138.5 863.9 22.3 Meta-xylene -47.4 138.8 866.3 48.0 Ortho-xylene -25.4 144.0 883.1 21.2 Total 100.0 ______________________________________ *Calculated thermodynamic equilibria at 850.degree. F. (454.degree. C.).
Principal sources of C.sub.8 aromatics mixtures are catalytically reformed naphthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources vary quite widely in composition but will usually be in the range of 10 wt % to 32 wt % ethylbenzene (EB) with the balance being about 50 wt % meta-xylene and about 25 wt % each of para-xylene and ortho-xylene.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation, although this is a costly operation. Ortho-xylene may be separated by fractional distillation, and it is so produced commercially. Para-xylene may be separated from the mixed isomers by fractional crystallization, selective adsorption, or membrane separation.
As shown above, the boiling point of ethylbenzene is very close to those of para-xylene and meta-xylene. As a result, complete removal of ethylbenzene from the C.sub.8.sup.+ aromatics mixture by conventional methods, e.g., distillation, is usually impractical. An ethylbenzene separation column may be used in the isomerizer-separator loop or the ethylbenzene may be converted catalytically in the isomerizer-separator loop.
In many processes for xylene isomerization, the conversion of ethylbenzene is not maximized because of the need to control the competing reactions which convert xylenes to less valuable compounds. Thus, when ethylbenzene is catalytically converted, the primary consideration for selecting the operating conditions is to minimize xylene losses from transalkylation of xylenes.
The present invention enhances the efficiency of producing xylenes for ethylbenzene conversion/xylene isomerization processes that are presently being used. The present invention also provides higher production of para-xylene in an ethylbenzene conversion/xylene isomerization process by using a two stage isomerization process to more efficiently isomerize xylenes.